The present disclosure relates to a solid-bowl screw centrifuge and to a process for operation of the solid-bowl screw centrifuge. The solid-bowl screw centrifuge includes a rotatable drum having a horizontal axis of rotation, which drum surrounds a centrifuging space which tapers at least in a portion. Further included is a screw which is arranged in the drum and is rotatable at a differential speed in relation to the drum. Also included is at least one solid-material discharge in the tapering region of the drum and an immersion disk on the screw. The immersion disk lies between a liquid feed and the solid-material discharge and subdivides the interior drum space or centrifuging space into a discharge space between the immersion disk and the solid-material discharge and a separating space between the immersion disk and the liquid discharge. The centrifuge also includes a device for admitting a gas to the separation space.
As noted above, the solid-bowl screw centrifuge, also known as a decanter, is provided with a rotatable drum. The solid-bowl screw centrifuge has a cylindrical portion and the tapering portion which is tapered conically. As also noted, arranged in the drum is a screw, which during operation rotates at a differential speed in relation to the drum.
In the decanter drum, an added suspension is separated into a liquid phase and a solid phase as a result of the centrifugal effect. At the same time, the solid material moves outward toward the inner wall of the drum, where it forms an annular layer. The differential motion between the drum and the screw causes conveyance of the solid material axially in the cylindrical part of the drum. In the conical part of the drum, radial conveyance is also required, counter to the acting centrifugal force.
Such a structural design is shown, for example, by DE 43 20 265 A1. In the design shown in this document, the distance between a weir for liquid discharge and a throttling disk can be changed by turning a threaded bushing. The accompanying changing of the outflow cross-section brings about a change in the liquid level in the centrifuging drum, so that an infinitely variable setting of this liquid level is possible by displacing the throttling disk.
It is known from DE 198 30 653 C1 that the liquid discharge of an open solid-bowl screw centrifuge occurs by a peeling disk. Downstream of the peeling disk there is a labyrinth seal to return product droplets to the peeling disk. According to this design, there is no need for sealing from the space outside.
A solid-bowl screw centrifuge in which the product space is sealed from the outside is disclosed by DE 102 23 802 B4. A barrier chamber with a barrier fluid feed in combination with an immersion disk and a siphon disk make it possible in this design for the centrifuging chamber to be sealed from the surrounding atmosphere. Although the design itself has proven successful, it is only conditionally suitable for the processing of products in which the solid material to be discharged or the phase to be discharged at the conical end is of relatively low viscosity.
See also DE 40 33 012 A1 and DE 30 22 148 A1.
It is also known, in the case of some types of decanters to measure the torque between the screw and the drum that is necessary for conveyance and to use this as an indicator of the amount of solid material located in the drum. If the decanter is appropriately equipped, for example, a two-gear drive or comparable drive, it is possible to regulate the differential speed in dependence on the measured torque in such a way that a largely constant degree of filling with solid material in the decanter can be set.
The mechanical conveyance by the screw is based substantially on force transmission by internal friction. The extent to which mechanical conveyance is possible, therefore, depends on the rheological properties of the solid-material composition.
FIG. 2 schematically illustrates the shearing motion in the solid material in dependence on an applied shear stress. One of the curves describes purely Newtonian behavior, in which there is a constant ratio between shear stress and shear rate, or viscosity, over the entire range under consideration. As a departure from this, the other curve shown comprises, for example, a primary shear stress, which first has to be exceeded before a shearing motion occurs. The greater the viscosity of a material, the better it can be mechanically conveyed. Conversely, difficulties occur in the discharge of solid material if the phase to be discharged is of a particularly low viscosity.
If the solid material has a low viscosity, under some circumstances it is possible to compensate for this by a correspondingly high differential speed. However, this method leads to different disadvantages, such as making it difficult for a decanter to be used for such separating tasks. Examples of these are extraction of pectins, extraction of lysine, thickening of surplus pulp, and beer recovery from spent yeast.
The present disclosure relates, for example, to the processing of these types of products.
Operational experience has shown that, in applications for the just-mentioned types of products, for example, it is scarcely possible in mechanical terms to achieve, in particular, the radial conveyance in the cone counter to the centrifugal effect.
To solve the problem of discharging relatively low-viscosity solid phases, it has been proposed in U.S. Pat. No. 5,244,451 to blow compressed air into the solid phase in the region of the cone, in order to reduce the average density of the solid phase. This has the effect that the solid material is forced inward and in the direction of the solid-material discharge openings at the conical end of the drum. Disadvantageous from aspects of such a structural design and process engineering are, in particular, the high pressures to be applied, which are 10 to 15 bar.
It is proposed in U.S. Pat. No. 3,885,734 to admit gas directly to the separation space. However, a disadvantage of this is that, although it may be possible after the pressurization for solid materials to be discharged, it is not possible to achieve a constantly improved discharge of solid material in stationary operation.
Against this background, the present disclosure relates to a solid-bowl screw centrifuge and a process for operating the solid-bowl screw centrifuge that makes it possible for relatively low-viscosity solids to be discharged.
The present disclosure relates to a solid-bowl centrifuge and a process for operating the centrifuge. The solid-bowl centrifuge includes a rotatable drum having a horizontal axis of rotation. The rotatable drum surrounds a centrifuging space and includes at least a tapering portion. A screw is arranged in the drum and the screw is rotatable at a differential speed in relation to the drum. At least one solid-material discharge is located in the tapering portion of the drum. An immersion disk is located on the screw. The disk lies between a liquid feed and the solid-material discharge and subdivides the centrifuging space into a discharge space located between the immersion disk and the solid-material discharge and a separating space located between the immersion disk and a liquid discharge. Also included is a device for admitting a gas to the separation space, wherein the liquid discharge is sealed from its surroundings in such a way that a liquid level R1 of a pond in a region of the separation space remains unchanged when pressurization occurs. The process for operating the solid-bowl centrifuge, comprises the following processing steps: providing a solid-bowl centrifuge that includes the following: a rotatable drum having a horizontal axis of rotation, the rotatable drum surrounding a centrifugal space and having a tapering portion; a screw arranged in the drum and rotatable at a differential speed in relation to the drum; at least one liquid discharge which is sealed from its surroundings; at least one solid-material discharge located in the tapering portion of the drum; an immersion disk on the screw, which disk lies between a liquid feed and the at least one solid-material discharge, the immersion disk subdividing the centrifuging space into a discharge space between the immersion disk and the at least one solid-material discharge and a separating space between the immersion disk and the at least one liquid discharge; and, a device to admit gas to the separation space; feeding a material to be centrifuged into the centrifuge via an inlet tube; operating the centrifuge; applying pressure to the separation space via a feed line wherein a level of a pond in a region of the separation space remains unchanged.
If a pressure other than ambient pressure is imposed on the separation or centrifuging space, i.e. the space in which the separation or decantation takes place, an inside diameter of the solid material that is dependent on the difference in pressure will be established in the conical discharge space. That is so since the liquid discharge is hermetically sealed from the ambient pressure in such a way that, in interaction with the baffle, the inside diameter or level of the pond in the region of the separation space remains unchanged when there is an increase in pressure in stationary operation. This is not the case in document U.S. Pat. No. 3,885,734 where the separation space is in connection with the ambient pressure at the liquid discharge via communicating tubes. Thus, when there is an increase in pressure there is a shift in the liquid level in the separation space, which has the consequence that the discharge of solid material is not permanently improved during operation. In accordance with the present disclosure, on the other hand, the sealing of the liquid discharge takes place by a peeling disk or by some other sealing means, for example, a hydrohermetic chamber, which is designed such that the pressurization does not lead to a shift in the level in the separation space.
If an inside diameter of the liquid phase is less than a diameter of the solid-material discharge of the drum, low-viscosity solid material is also conveyed out of the drum. If the inside diameter is greater, there is no solid-material discharge. In order to carry such solid material away, it is generally necessary to apply a pressure of 0 to 10 bar, or, for example, 0.5 bar or more, or, for example, 0.5 to 5 bar, to the separation space.
The device for admitting a gas to the separation space has a feed line into the separation space, which during operation opens out into the separation space on a radius that is less than the radius of the liquid level during operation.
The gas may be compressed air, which may be sterile air, or, for example, nitrogen.
The present disclosure also relates to optical measurement of the torque between the drum and the screw, which is a measure of the degree of filling with solid material in the decanter. The optical measurement signal is fed to the pressure control unit and evaluated and used as a control signal for a setpoint value of the imposed pressure. The differential speed between the screw and the drum thereby remains constant. It is possible, in accordance with the present disclosure, to dispense with a secondary drive for changing the differential speed. Rather, this remains constant. The actual control of the process, on the other hand, takes place in a way by variation of the pressure in the separation space. The separation space is measured by a further line.
In an embodiment of the present disclosure, the amount of solid-material discharge is controlled or regulated by variation of the pressure in the separation space.
Embodiments, according to the present disclosure, include, for example, the following advantages. Monitored metering of the conveyance of solid material in the decanter even in the case of solid-material compositions which, in mechanical terms, cannot be conveyed, or only with difficulty. A possible cost saving obtained by dispensing with the secondary drive. No influence on the conveyance of solid material by the so-called idling torque, which is dependent on the high differential speed. Rather, it is conceivable, according to the present disclosure, for the differential speed, and consequently the idling torque, to be kept constant. The solid material can be drawn off over a small diameter.
The admission of a gas to the separation space offers an optional way of imposing a protective gas on the sedimentation pond.
Other aspects of the present disclosure will become apparent from the following descriptions when considered in conjunction with the accompanying drawings.